I was thinking of playing a confederacy of the Freemasons, the Amish, and the Quebec Liberation Front along the Great Lakes.
The Freemasons would have power because they maintain the locks keeping the rising seas from salinating the lakes. They have always guarded the knowledge of building walls using only human power, and that is suddenly as valuable as it was 600 years ago.
The Amish rose to power with the skyrocketing fuel prices. Suddenly, they went from being mocked to being revered for their preservation of traditional agriculture. They were responsible for feeding what remained of the East Coast. Of course, with power comes corruption, and they are twisted beyond what most would recognize from the turn of the 21st century. They are now ruled by the Council of Gabriel, the twelve Patriarchs of the "original tribes", and they are very strict about who they allow into positions of power.
The Quebec Liberation Front is used mostly to both pacify and antagonize the sole superpower to the north. Their extensive networks of spies, saboteurs, and propagandists have so far kept the Canadian Empire from seizing most of the waterways, although the Canadian Navy is the de facto ruler of Lake Superior.